Opinions vary on how genes affect mortality rate. A common view is that genetic variation primarily affects the rate of increase in mortality with age, but recent data suggests that genetic variation primarily affects the initial mortality rate. The proposed research intends to resolve this ambiguity. It will determine for a series of natural populations of grasshoppers and for recently published laboratory populations of invertebrates whether genetic variation affects longevity through variation in the initial mortality rate or the rate at which mortality changes with age. Populations of genetically distinct Melanoplus sanguinipes grasshoppers from the Sierra Nevada, California will be used to represent natural variation in aging. M. Sanguinipes along an altitudinal gradient exhibit ordered differences in adult longevity under sheltered conditions. These differences in survival reflect genetic variation for senescence. The research will determine whether this variation is due primarily to differences in initial mortality rate or in the rate of change in mortality. The project will construct, under common sheltered conditions, large life tables for male cohorts from each of five populations along the gradient. Life tables will be constructed in each of two thermal conditions to control for gene-by- environment interactions. Second generation laboratory offspring will be studied to eliminate variation due to maternal effects. To evaluate the effects of genetic variation in laboratory populations, the research will compile demographic dat for genetic variants of Drosophila melanogaster and Ceonorhabditis elegans published since 1990 and reanalyze these life tables with maximum likelihood and resampling methods. The program addresses a fundamental question about aging in both human and animal model systems: How do genes affect mortality rate? The work will help bridge the demographic and molecular approaches to aging. From this approach we may come to understand the causes for senescence and discover avenues to manage mortality and morbidity.